1. Field of the invention
This invention relates generally to valves for throttling and blocking the flow of solid particles and solid particles mixed with gas streams. More specifically, this invention relates to slide valves.
2. Description of the prior art
Slide valves find routine application in the throttling of gas streams containing solid particles and the throttling or complete blockage of solids flow. Familiar applications for slide valves include fluid catalytic cracking units, fluid coking units, and fluidized iron ore units. Service conditions for slide valves are typically harsh and operate at temperatures that can vary from atmospheric to over 1600.degree. F., with temperatures typically ranging from 800 to 1400.degree. F. Pressure containment for the valve body and components ranges from atmospheric conditions to over 250 psig. Pressure drops created by flow restriction across the valve will typically range from less than 1 psi to 50 psi. U.S. Pat. Nos. 3,726,306 and 3,964,507 show typical slide valve arrangements that have a sliding disc retained in a slot for sliding movement across an orifice that serves as a variable flow path opening for throttling or occluding flow.
Slide valves are not ordinarily used to provide gas tight sealing, but are understood to be throttling or flow blocking devices. Slide valve arrangements for blocking or throttling flow have traditionally used a single disc to cover the orifice opening. Throttling valves have an orifice opening which offers a reduced flow are relative to the nominal line size in which the valve is placed. An orifice plate fixed with respect to the valve body defines the orifice opening for most valve arrangements. In throttling operations, the orifice opening ordinarily provides a flow area substantially less than the flowing diameter of the line across which the valve regulates flow. Sizing of the orifice opening usually restricts its open flow area during operation to about 50 to 80% of total area of the orifice opening.
It is known to use different disc or orifice configurations to program the flow control of the response of the valve through the range of disc movement. The use of triangular orifice openings in a slide valve is known from U.S. Pat. No. 5,368,276. In this valve arrangement, the triangular orifice opening is defined by the movable disc and moves with respect to the orifice opening defined by the valve.
As the disc changes positions across the opening of the orifice, downstream flow characteristics of the fluid and particles is affected. In order to avoid or reduce erosion on downstream piping, it is highly desirable to keep the flow of particles and fluid centered in the orifice opening. As the size of the orifice opening changes with position of the disc during throttling, the centralization of the flow stream with respect to valve body and piping will change. Maintaining the flow as centralized as possible through the valve minimizes erosion downstream of the disc and orifice which by contact of catalyst the surfaces of the valve body.
The combination of high velocity gases and elevated temperatures can lead to severe erosion problems in slide valves. Elevated temperatures significantly reduce the physical strength and hardness of the valve material. Erosion substantially reduces the operating life of valves when it occurs. When erosion becomes severe enough, valve components must be replaced--usually requiring the shutdown of expensive operating equipment.
One of the most severely erosive environments is encountered when regulating the discharge of flue gases from an FCC unit. This slide valve in a flue gas operation regulates the pressure of the FCC regenerator by throttling the discharge of exhaust gases created by the combustion of coke from catalyst particles. This slide valve operates in combination with a series of fixed orifices located in a chamber that reduces the pressure of the flue gas from a range of from 15 to 30 psi to atmospheric conditions. Fixed orifices usually account for half of the pressure drop imposed by the flue gas. The remaining approximately other half of the pressure drop occurs across the slide valve and varies as pressure is adjusted through the throttling of the slide valve. Flue gas typically leaves an FCC unit at elevated temperatures of over 1200.degree. F. and as high as 1500.degree. F. The loading of solid particles or catalyst in the flue gas is relatively light--typically in the order of 0.05 lbs/ft.sup.3. However, the velocity of flue gas through the valve approaches sonic velocity and is usually as high as 0.5 to 0.75 mach. The high velocity and high temperature of the gas and catalyst can result in severe erosion on the surfaces of the slide valve.
A number of slide valve arrangements are taught with specific objective of better adapting the valve to the erosive environment of flue gas and particulate solids. Valve designs that include components and arrangements for suitability to erosive environments include U.S. Pat. Nos. 2,217,834; 2,614,789; 2,636,712 and 3,370,610.
It is known from U.S. Pat. No. 4,253,487, the contents of which are hereby incorporated by reference, to center flow in a slide valve to reduce erosion. U.S. Pat. No. 4,253,487 teaches the use of two opposing discs in a slide valve that are retracted from each other to define a central opening that remains centered across the flow area of the orifice as the open area of the orifice changes through repositioning of the discs. While the dual disc slide valve keeps the central section of the flow centered with respect to the axis of movement of the discs, it adds the expense and complexity of supplying and operating a valve with dual discs. The opening provided by the dual discs also has fixed sides in the direction transverse to disc movement. The constant width of the orifice opening in a direction normal to direction of the disc movement limits its ability to reduce erosion with respect to the portion of the valve adjacent the constant width section.